I'm afraid I won't be able to help you much more, Roger. A couple of thoughts, however:
I couldn't find anything about the 6951st in my reference stuff, nor anything pertinent on the 'net. This is entirely speculation, but I am inclined to think that it was an installation, rather than a unit. A "prisoner of war temporary enclosure" does not strike me as a unit designation. (At least it is unlike anything with which I am familiar.) I suspect it was an installation, manned by personnel from such units as the 159th. Unfortunately, I am at a complete loss on the "doomsday" references.
As for the 69th Amphibian Tractor Battalion, according to Stanton the 69th Tank Battalion was redesignated the 69th Amph Trac Bn (rather than one unit "absorbing" the other, as you had alluded) on 11 July 45. Said reference contains no further info, save the notation that the Battalion arrived NY 6 Mar 46, and was inactivated 8 March 46 at Camp Kilmer, NJ. Redesignation of the unit, you will note, was following the end of hostilities (which ceased formally the first week of May). Their duties, then, from redesignation until they left Europe, would have been as a part of the Allied occupation forces.